1. Field of the Invention
The invention relates to an arrangement with at least one superconductive cable and a first cryostat surrounding the cable for conducting a first cooling agent, wherein the cryostat is composed of two pipes of metal which extend coaxially and at a distance from each other, and a thermal insulation enclosed between the pipes, and wherein the cryostat surrounds over its entire length a hollow space in which the cable is arranged and through which the first cooling agent is conducted during operation of the arrangement, wherein the first cooling agent used is a liquid or gaseous cooling agent cooled to a temperature of 39K or lower, and wherein a second cryostat for conducting a second cooling agent is arranged coaxially around the first cryostat and at a distance from the first cryostat, wherein the second cryostat is also composed of two pipes of metal extending coaxially and at a distance and a thermal insulation enclosed between the pipes, and wherein a liquefied gas having a temperature of 112K or lower is conducted through the second cryostat during operation of the arrangement.
Such an arrangement is disclosed, for example, in DE 19 48 520 A1.
2. Brief Description of the Prior Art
A superconductive cable has electrical conductors of a material which changes over into the superconductive state at sufficiently low temperatures. The electrical direct current resistance of a conductor constructed accordingly is zero at sufficient cooling as long as a certain voltage—the critical voltage—is not exceeded. Suitable materials are, for example, oxidic materials on the basis of rare earths (ReBCO), particularly YBCO (Yttrium-barium-copper oxide), or BSCCO (bismuth-strontium-calcium-copper oxide). Sufficiently low temperatures for bringing such a material into the superconductive state are, for example, between 67K and 110K. Suitable cooling agents are, for example, nitrogen, helium, neon and hydrogen, or mixtures of these materials. Since the indicated temperatures are significantly above the absolute zero point, of −273.16′C.=0K, conductors of the corresponding materials are referred to as high temperature superconductors (HTS-conductors). This is also true for other superconductive materials, for example magnesium diboride, which change over into the superconductive state at temperatures of about 39K or lower, which are also significantly above 0K.
EP 2 234 122 B1 discloses an arrangement in which a superconductive cable is arranged in a cryostat. The cryostat is composed of two pipes of metal which are arranged concentrically and at a distance from each other and which include therebetween a thermal insulation constructed as a vacuum insulation. Used as superconductive materials are the oxidic materials on the basis of rare earths already mentioned above. For example, liquid nitrogen, which can be kept at a sufficiently low temperature with a conventional vacuum insulated cryostat over a longer period of time, is used for cooling the cable. Liquid nitrogen cannot be used for low temperatures which are below 63K because it changes into a solid state at these temperatures. In addition, it is increasingly difficult to keep the cooling agent which has the appropriate low temperature at this temperature for a longer period of time.
JP 2002352645 A describes a system with a superconductive cable in which magnesium diboride is used as superconductive material. The cable is surrounded by a cryostat composed of two pipes arranged concentrically relative to each other with an insulation arranged in the pipes. In addition, the space between the two pipes is evacuated. The outer pipe is surrounded by an electrical insulation onto which are wound two layers of steel bands as mechanical protection.
The aforementioned DE 19 48 520 A1 describes an arrangement with a pipe line for frozen cables with two concentrically arranged cryostats which are both composed of two concentric pipes having a vacuum therebetween. Cooling agents having different temperatures are conducted through the two cryostats, for example, liquid helium is conducted through the inner cryostat and for example, liquid nitrogen is conducted through the outer cryostat. Suitable materials for the cable constructed as a superconductive cable are, for example, alloys of niobium and titanium, or niobium and zirconium, as well as a niobium-tin compound are mentioned.